This invention relates to ultrasonic diagnostic imaging and, in particular, to the segmentation (identification) of ultrasonic signals relating to tissue and ultrasonic contrast agents.
Ultrasonic contrast agents are in widespread use for the detection and diagnosis of disease. The current generation of ultrasonic contrast agents are comprised of tiny microbubbles, frequently encapsulated in soluble substances such as lipids, which are infused into a patient""s bloodstream. These contrast agents have the property that, when insonified by an appropriate level of ultrasonic energy, the microbubbles will respond nonlinearly and return echo signals which contain nonlinear components of the linear transmit wave. These nonlinear components manifest themselves most strongly at harmonic multiples of the transmit frequency and hence the agents are generally referred to as harmonic contrast agents. Harmonic contrast agents advantageously provide excellent segmentation in relation to other echo signals because of their strong manifestation at the harmonic frequencies.
Since harmonic contrast agents travel through the cardiovascular system they are ideal for improving the diagnosis of blood flow and perfusion. Harmonic contrast agents are used to diagnose obstructions in the coronary arteries as described in U.S. patent application Ser. No. 09/645,872 or to diagnose myocardial perfusion as described in U.S. Pat. No. 5,833,613. The tiny microbubbles afford excellent patient safety for, unlike the problems that can result from large air emboli, the microbubbles are filtered from the bloodstream by normal bodily functions. A consequence of this is that the contrast agents, once infused into the body, do not remain there indefinitely but are rapidly removed. Hence it is necessary to perform contrast exams efficiently and expeditiously while the agent is in the bloodstream. It is a general practice prior to the introduction of the agent for the examining clinician to image the organ or region of the body which is the subject of the exam, to find the best acoustic window, probe inclination, and image planes by which the most diagnostic images can be acquired. Once the clinician has acquired these xe2x80x9cbaselinexe2x80x9d images and formulated the specific technique for acquiring the images, the agent is infused and the desired contrast images quickly and efficiently acquired.
These baseline images, acquired as they are before any contrast agent has been introduced into the patient""s bloodstream, should be free of any appearance of the agent. For instance, the contrast agent will often be displayed in color in the image, with the other structures in the body depicted in grayscale. In such a procedure the baseline images should appear free of any color in the images. But often these baseline images will be contaminated by artifacts resulting from noise, probe motion, or other sources. The clinician""s natural response to seeing these artifacts is to turn down the transmit power or the receiver gain or to increase the noise rejection threshold in the images until these artifacts disappear. This, however, can reduce the sensitivity of the ultrasound system to contrast agents and thus degrade the diagnostic capability of the exam. It is desirable to minimize these artifacts in the image so that highly sensitive contrast images may be acquired without contamination by artifacts appearing to be contrast.
In accordance with the principles of the present invention an apparatus and method are provided for segmenting ultrasonic contrast image signals, thereby enabling the production of more sensitive and artifact-free ultrasonic harmonic images. Ultrasonic energy is transmitted and acquired so that nonlinear signals may be separated by the pulse inversion process. The received signal spectrum is analyzed to measure the relative power of linear and nonlinear components of the echo signal. The distribution of the received signal power and the ultrasound system noise threshold or sensitivity level are combined to produce a threshold against which the nonlinear signal level is compared. Signals that exceed the threshold are displayed in the nonlinear signal display format, and signals that are below the threshold are displayed in the linear signal format. Alternatively, signals falling below the threshold may not be displayed at all. This segmentation reduces the artifact level in both harmonic contrast and tissue harmonic images. The present invention also makes possible the display of a nonlinear signal which has been compensated for the effects of linear or weakly nonlinear tissue clutter and electronic noise.